The present invention generally relates to semiconductor devices, and in particular to an improvement of high-frequency power semiconductor devices such as permeable base transistors or static induction transistors having a vertical channel of carriers, but not limited to these.
Permeable base transistors or static induction transistors are promising semiconductor devices that are capable of operating at high frequencies with a large output power. On the other hand, these devices have various fabrication problems because of the peculiar device structure.
FIG. 1 shows the essential part of a permeable base transistor disclosed in the U.S. Pat. No. 4,378,629.
Referring to FIG. 1, the transistor comprises an n.sup.+ -type GaAs substrate 11 on which an n-type GaAs layer 12a is grown. The GaAs layer 12a is made unconductive by the ion implantation of protons except for a part 13 that is used for the emitter of the permeable base transistor. Thereby, the rest of the GaAs layer 12a acts as the device isolation region.
On the GaAs layer 12a, a tungsten base layer is deposited with the thickness of less than about 2000 .ANG., and the tungsten layer is patterned by a photolithographic process to form a base structure 14 wherein a number of fingers 14a are formed to extend parallel with each other along the top surface of the emitter region 13. Further, another n-type GaAs layer 12b is deposited on the GaAs layer 12a to bury the base structure 14, and a groove 12c is formed in the GaAs layer 12b to expose a part of the tungsten base structure 14.
The GaAs layer 12b is rendered unconductive by the ion implantation of protons except for a collector region 15 that is located in the layer 12b to oppose the emitter region 13 across the base structure 14, and a base electrode 17 and a collector electrode 18 are provided respectively in contact with the base structure 14 and the collector region 15. It should be noted that the base electrode 17 is provided along the sloped surface of the groove 12c to contact with the base structure 14 exposed at the bottom part thereof. Further, an emitter electrode 16 is provided on the bottom surface of the substrate 11.
Typically, the base structure 14 has a thickness of less than 1000 .ANG. as already described, and the fingers 14a are separated from each other by 0.32 .mu.m for example. The electrons are transported vertically from the emitter region 13 to the collector region 15, passing through a channel region 20 formed between adjacent fingers 14a of the base structure. Thereby, the electrons experience the electric field that is formed by the base structure 14 in response to a base voltage applied thereto.
As the thickness of the base structure 14 in the vertical direction can be easily reduced to less than 2000 .ANG. at the time of deposition of the tungsten layer, the transistor of FIG. 1 has an excellent high frequency characteristic. Further, the device is characterized by the large number of channels that is formed between the fingers 14a of the base structure 14. Thereby, a large output power can be obtained while maintaining the small lateral size of the base structure 14. About the permeable base transistor, reference should be made to IEEE MTT-S Digest, 1988, which is incorporated herein by reference.
The static induction transistor has a structure substantially identical with that of FIG. 1 except that the base structure 14 is formed of an p-type semiconductor material, and the emitter region 13, the collector region 15 and the channel region 20 are all formed from an undoped or lightly-doped n-type semiconductor material. About the static induction transistor, reference should be made to Nishizawa et al. (Nishizawa, J. I., et al., IEEE Transactions on Electron Devices, vol.ED-22, No.4, p.185, 1975), which is incorporated herein by reference.
In the foregoing permeable base transistors or static induction transistors, there exists a problem in that the fabrication of the device is difficult because of the complex device structure, particularly the base structure 14, and associated therewith, there is another problem in that impurities tend to be incorporated in the critical part of the device during fabrication. It should be noted that the base structure 14 has to be embedded between the GaAs layer 12a and the GaAs layer 12b, after depositing and patterning the tungsten layer on the GaAs layer 12a. This means that the device has to be removed from the deposition apparatus to the atmospheric environment for photolithographic patterning. Thereby, contamination of the surface of the layer 12a by atmospheric impurities such as carbon or oxygen is inevitable and such impurities cannot be removed perfectly even after a meticulous cleaning process.
FIG. 2 shows such impurities 19 in the case of the device wherein the base structure 14 is formed directly on the top surface of the GaAs layer 13, and FIG. 3 shows the impurities 19 in the device wherein the base structure 14 is formed in a groove 13a formed on the top surface of the GaAs layer 13. In these examples, the impurities 19 exist in the channels 20 of the electrons, and because of this, the passage of the electrons through the channels 20 is inevitably affected by the impurities. These impurities may for example form the deep impurity levels in the channels 20 that in turn cause the modification of the band structure such as the formation of depletion regions within the channels 20. Further, the impurities may cause the scattering of the carriers passing through the channels 20.